1. Field of the Invention
The present invention relates to an anti-lock brake system which prevents wheel lockup during braking and thus avoids skidding, and specifically to an automotive anti-lock brake control system which is capable of making a self-diagnosis on the trouble of an electric motor adapted to rotate a return pump employed in the system.
2. Description of the Prior Art
In recent years, there have been proposed and developed various anti-lock brake systems which are designed to prevent skidding and thus provide maximum effective braking. On more later-model cars with an anti-lock brake system, a self-diagnostic system is often incorporated in the anti-lock brake system, to make a self-diagnosis on motor wire troubles such as a loose motor wire connector of an electric motor of a return pump, often called "ABS pump," fluidly disposed in a hydraulic actuator of the anti-lock brake system. As is generally known, the typical anti-lock brake system uses a reservoir tank as well as a return pump. The return pump is adapted to reduce the hydraulic pressure in a wheel-brake cylinder for a certain road wheel selected for the purpose of skid control. The reservoir tank is fluidly disposed in a brake line connected to both the inlet port of the return pump and the wheel-brake cylinder for temporarily accumulating the brake fluid exhausted from the cylinder as a result of reduction in the pressure in the cylinder. The return pump included in the ABS is usually driven by way of an electric motor. In the event that the motor wire connector of the motor-driven return pump is disconnected or the motor wire itself is burnt out during operation of the ABS, the anti-skid control terminates in an abrupt manner (for a brief moment such as 0.1 sec or less). Thus, it is necessary to make a self-diagnosis upon the abnormality of the motor associated with the return pump. On conventional anti-lock brake systems with a self-diagnostic system which makes a self-diagnosis on the abnormality of the motor of a return pump, the anti-lock brake system is generally designed to terminate the system operation and to permit normal application of the brakes, immediately when the self-diagnostic system decides the presence of abnormality of the electric motor of the ABS pump during operation of the ABS. As can be appreciated, if normal application of the brakes restarts as soon as the system decides the motor of the ABS pump is in trouble during the skid control, a satisfactory anti-lock brake control action cannot be provided and thus a satisfactory convergence of a skid of the skidding less-traction wheel cannot be accomplished due to a rapid transition from skid control to normal braking.